1. Field of the Invention
The present invention relates to D.C. power supplies employing controlled ferroresonant transformers and more particularly to the provision of failure alarms in such systems.
So-called ferroresonant power transformers consist of an iron core upon which separate primary and secondary coils are wound and separated from each other by magnetic shunts in the core. The magnetic shunts cause some of the flux induced by the primary to be diverted from the secondary resulting in a certain amount of isolation between the primary and secondary windings. The shunts also allow the magnetic flux levels in the respective core sections associated with the primary and secondary to be different in both amplitude and phase. The secondary of such a transformer is connected across a suitably selected capacitor so that the secondary and the capacitor form an oscillator circuit, commonly referred to as a "tank circuit". When sufficient A.C. voltage is applied to the primary of such a transformer, the transformer abruptly assumes a condition called ferroresonance, in which oscillations in the tank circuit cause the secondary transformer core section to experience A.C. flux saturation.
When the secondary core section is saturated a portion of the voltage provided across the secondary can be tapped off, rectified and filtered to produce a source of relatively constant D.C. voltage for supplying a D.C. load. Within limits, changes in the A.C. voltage and/or frequency input to the transformer change only the saturation level of the core and produce relatively small changes in output voltage to the D.C. load. Increasing the D.C. load, within limits, drains energy from the tank circuit and reduces the saturation of the core but also results in only a small change in D.C. output voltage.
Thus, so long as the transformer secondary core section remains saturated, the D.C. output voltage remains relatively constant regardless of fluctuations in the load and in the power supply voltage and frequency; however, if the input voltage or frequency is reduced below a predetermined value, or if the load increases sufficiently, the energy in the tank circuit is not sufficient to maintain the transformer core saturated and the output voltage to the load drops precipitately.
The disadvantages of the ferroresonant power transformer arrangement referred to are that the degree of D.C. output voltage regulation is dependent upon the magnetic characteristics of the transformer core material and thus cannot be controlled with precision; the output voltage is determined by the number of turns tapped from the transformer secondary and cannot be easily changed; the output voltage from the rectifier is dependent upon changes in the frequency of input power to the transformer; and, the load at which the output voltage drops precipitately is highly sensitive to changes in the transformer input voltage.
The foregoing disadvantages were largely overcome by the development of so-called controlled ferroresonant power supplies. These included ferroresonance control circuitry associated with essentially the same transformer and tank capacitor arrangement referred to previously. The ferroresonance control circuitry enabled the impedance of the tank circuit to be controllably altered thus permitting the tank circuit to variably simulate saturation of the secondary core section.
The ferroresonance control circuits provided for a variable impedance in parallel with the tank capacitor to alter the impedance of the tank circuit. The variable impedance was commonly formed by an inductor and an electronic switch which switched the inductor in and out of the tank circuit electronically in a controlled fashion to controllably alter the impedance of the tank circuit to simulate saturation of the transformer core. By thus simulating saturation of the transformer core so that the voltage across the tank capacitor was normally less than the minimum which would have existed with the heaviest expected load and the lowest expected input voltage and frequency, the output voltage was controlled electronically and could be made to be essentially independent of input voltage and frequency, load current and the magnetic characteristics of the core itself.
In a typical application, two or more controlled ferroresonant rectifier units have been connected across a load in parallel with a battery. The rectifier units operated to share the load and maintain the battery charge level. In the event of a power interruption to the rectifier units the battery provided a temporary power supply for the load. If one of the rectifier units malfunctioned, the load was distributed among the remaining rectifier units. In the latter event a rectifier failure alarm was produced so that the existence of the malfunction was brought to the attention of the equipment user.
2. The Prior Art
Controlled ferroresonant power supplies were provided with various accessory circuits which individually sensed different failure modes of the power supplies and produced failure alarm signals. These included low current alarm circuits and high voltage shutdown circuits.
The high voltage shutdown circuits sensed the existence of a voltage across the rectifier unit output terminals which exceeded a predetermined set point value. This condition was indicative of a rectifier unit failure. The high voltage shutdown circuits commonly interrupted the supply of power to the rectifier unit by means of a resettable circuit breaker.
The low current alarm circuits were employed to determine when the rectifier unit output was minimized, thus indicating a failure of some sort in the unit. To accomplish this the low current alarm circuits sensed the rectifier unit output current and when that current reached a predetermined minimum value (generally 0.5% of full load current) a rectifier failure alarm was produced. Current sensing was typically accomplished by the use of a current shunting device from which a signal was derived and amplified to control operation of an alarm producing device. These circuits produced failure alarms when load levels were low but without any actual rectifier malfunction occurring. The low current alarm circuits had to be constructed to respond to arbitrary low current levels because of the difficulty in sensing and responding to extremely small rectifier output currents.